1. Field of the Invention
The present invention relates to an apparatus and method for controlling an injection molding device and, in particular, to an apparatus and method for controlling an injection molding device in a manner that results in the production of a molded part, such as a substrate for an optical medium, having substantially no residual stresses remaining therein.
2. Description of the Prior Art
An optical medium is a data storage element used to store recorded music, recorded video, and/or digital data. The medium comprises a thin, wafer-like substrate, or disc, coated with thin-film dielectric, magneto-optic and metallic reflective layers and a protective polymer overcoat. The substrate may be glass or may be formed by injection molding from a molten mixture of polymeric materials, such as a polycarbonate plastic material.
In order to be useful in certain applications, for example as an erasable (or "rewritable") optical storage medium, an injection molded part that forms the substrate of the medium must meet, at the minimum, ANSI functional specifications that mandate less than twenty nanometers (20 nm) retardation measured with a double pass, focused beam instrument. Lesser retardation levels, i.e., on the order of ten nanometers (10 nm) or less, measured with a double pass, linear polarization, focused beam instrument, are even more preferred, because of the concomitant increase in signal-to-noise ratio provided thereby. However, discs manufactured by an injection molding device controlled in accordance with known manufacturing schemes have average birefringence levels that are on the order of between thirty and thirty-five nanometers (30 to 35 nm). These levels of birefringence are believed due to the presence of residual stresses in the injection molded substrate.
The origin of these residual stresses may be understood when one considers the details of the fabrication of a substrate using a typical injection molding device. With reference to FIG. 1A, shown is a highly stylized representation of the major components of a conventional injection molding device generally indicated by the reference character A, presently used to fabricate, by injection molding, a substrate for an optical disc. Representative injection molding devices as shown in FIG. 1A include those manufactured by Technoplas Inc. and Klockner Ferromatik GmbH. These two-mentioned devices are in a class of injection molding devices which differ from other classes of injection molding devices because of the presence of a hydraulically operated, fully variable, clamping ram.
The injection molding device includes a fixed mold member F and a movable mold member M. The fixed mold member F has a material inlet passage P extending therethrough. The passage P may be interrupted by the insertion into the passage of a device known as a gate closure G. It should be understood that although the closure is shown throughout this application as implemented by a mechanical member, the gate closure can be alternatively implemented by presenting a constriction within the passage P.
The mold member M is moved with respect to the fixed member F by an actuating ram R. When acting in a first direction F.sub.1 the ram R imposes a clamping force on the movable member that urges the members together whereby they cooperate to define a mold cavity C therebetween. In FIG. 1A the mold members M and F are shown in the clamped position, whereby the cavity C is defined. The volume of the cavity C is governed by the relative spacing between the mold members F and M. The action of the ram R in the opposite direction F.sub.2 imposes an unclamping, or opening, force on the movable member, thereby causing the member M to separate from the member F to open the cavity C. Actuating hydraulic fluid is applied to the ram R from a pressurized fluid source S through a pressure line L. A control valve V is usually disposed in the line L. A molten mixture of polymeric material is supplied to the cavity C from the injector I, the material passing into the cavity C through the passage P in the fixed mold member F.
The conventional injection molding cycle is divided into several separate phases: (1) filling; (2) packing; (3) holding; and (4) cooling. The overall molding cycle extends for a predetermined molding cycle time. The variations in cavity volume, cavity pressure, and the magnitude of the clamping force during the phases of the conventional molding cycle may be understood from FIG. 1B.
Filling is the rapid volumetric filling of the mold cavity C. During the filling operation the injector I fills the cavity by rapidly forcing molten polymeric material thereinto. From FIG. 1B it is seen that during the filling phase the cavity volume and the imposed clamp force are both constant, but the cavity pressure rapidly increases. Packing is the slow pressure driven flow of molten polymeric material into the mold cavity C to insure complete filling thereof. During the packing phase the cavity volume, the imposed clamping force and the cavity pressure all remain substantially constant.
During the holding phase the molten polymeric material in the cavity C begins to cool, and continued slow pressure driven flow of molten polymeric material into the mold cavity compensates for the cavity volume reduction due to shrinkage. The holding phase continues until the gate G is closed or frozen, thus blocking the passage P from the injector I and thereby isolating the cavity C therefrom. During the cooling phase no further molten polymeric material can enter the cavity, and the part in the cavity C cools and continues to shrink. This shrinkage accounts for the decrease in the cavity pressure during the cooling phase, as seen in FIG. 1B. The cooling phase continues until the end of the predetermined molding cycle time, when the opening force is applied to the movable mold member M and the mold is opened. Using the conventional procedure as here outlined produces frozen-in residual stresses. If the part is used as a substrate for an optical disc, the frozen-in residual stresses create an unacceptably high level of birefringence in the finished disc.
An alternative molding control is shown in the dashed portion of the cavity-volume curve in FIG. 1B. In this alternate molding control method the molten polymeric material is initially injected into the mold cavity C at an injection pressure (not specifically illustrated in FIG. 1B) that exerts a force on the movable mold member M that is greater than the clamp force. This causes the movable mold member M to open during the filling and the early portion of the packing operations (thus the reason for the increase in cavity volume). This opening of the mold is termed "breathing" of the mold. As the injection pressure is decreased, the cavity volume returns to its original volume.
An alternative molding control method is discussed in the article by J. Greener, "Producing Precision Parts at Injection Speeds", Plastics Engineering, June 1987. This method is termed "hybrid molding" and the variations in cavity pressure, cavity volume and clamp force during the molding time cycle using such a method are shown in FIG. 1C. In this method the clamp force also remains fixed throughout the cycle. The introduction of molten material from the injector (at an injection pressure that initially exerts a force on the movable mold member M that is greater than the clamp force) causes the cavity volume and the cavity pressure to increase. The gate G is then mechanically closed or frozen. As the part cools, the clamp force remains constant and the cavity volume decreases as shrinkage occurs. Since cavity pressure is not measured and controlled, it is erratic. Erratic cavity pressure may be due to undetected disturbances in the systems, for example, frictional variations in the mold and the clamping system, changes in the hydraulic system supplying the ram and creating the clamp force, and variations in the physical properties of the polymeric material being molded. This causes variations in the resultant molded parts. This method, while an improvement over the two previously described methods (FIG. 1B), still produces parts that are unsatisfactory for precision optical media substrates.
In view of the foregoing it is believed advantageous to provide a controller apparatus and a method for controlling the operation of an injection molding device that results in the production of molded parts having lesser residual stresses and lesser birefringence.